1. Field of the Invention
This invention relates to the field of voltage controlled oscillators. More particularly, but not exclusively, this invention relates to voltage controlled oscillators of a type suitable for use in generating clock signals used to drive digital circuits.
2. Description of the Prior Art
As digital integrated circuits scale down in size and increase in speed it becomes increasingly difficult to provide noise and jitter free clock signals. Phase locked loop systems including voltage controlled oscillators are often used in the synthesis of clock signals in digital systems. Voltage controlled oscillators generate an output signal with a frequency that is dependent upon an input control signal. The input control signal is a particularly sensitive node within the system as it is typically left floating for a large proportion of the time and so small noise voltages can produce large frequency shifts, e.g. a one Volt change in the input control voltage might produce a 100 MHz change in the oscillator output signal frequency. The input control signal may be either single ended or differential.
Differential control voltage systems are more complex to design and fine tune than single ended systems. Differential systems also tend to consume more power due to the additional and larger circuitry they contain and have more failure mechanisms. However, differential systems typically have a greater input control signal range than single ended system and this can be an advantage as a given frequency range of the voltage controlled oscillator can be provided using a wider input control signal range so making the oscillator output less prone to noise on the input control signal. An example of a complex differential phase locked loop system can be found in ISSCC Digest of Technical Papers, February 1995, page 112.
Single ended control voltage systems are generally more popular than differential systems due to their simplicity and proven performance. Typically the input control voltage is supplied to the gate of a single FET (the gate connection allows the voltage to float as is needed) which acts as a voltage to current converter. The resulting current then acts to control a current controlled oscillator. A problem with such systems is that the voltage-to-current FET conducts substantially zero current until the input control signal passes a threshold level and at the other end of the voltage range saturation effects degrade the linear response. This limits the control voltage range, particularly as long channel FETs are used that tend to have higher threshold voltages. Examples of single ended voltage controlled oscillators can be found in the IEEE Journal of Solid State Circuits November 1998, page 1636; October 1997, page 1516; April 1997, page 584; April 1995, page 383 and November 1992, page 1599.
The problem of a reduced control voltage range due to threshold voltage effects can also occur differential systems. The threshold voltage problem is made worse by reducing the operating voltage of the oscillator in an effort to reduce power consumption a since the input control signal range lies between the substantially fixed threshold voltage and the reduced operating voltage level.
The present invention has the object of addressing the above described problems.